A conventional automatic transmission is known which includes a mechanical oil pump and an electrical oil pump and in which a hydraulic pressure is supplied from the electrical oil pump to a gear corresponding to a starting shift position during an idle stop control.
However, if a gradient of a road surface on which a vehicle is stopped is large, the inclination of an oil surface in an oil pan is large and a suction port of an oil strainer comes out of oil, whereby air is sucked into the electrical oil pump together with oil. If air is introduced into the electrical oil pump, a hydraulic pressure may not be sufficiently supplied from the electrical oil pump to the gear corresponding to a start position. If the vehicle is stopped on an uphill road surface in a traveling direction and an idle stop control is executed in such a case, it takes a long time until the gear is engaged and the vehicle may move backward during a time until the gear is engaged when a return is made from the idle stop control.
Contrary to this, in JP2002-47962A, the idle stop control is prohibited when a gradient of a road surface on which a vehicle is stopped is large, whereas the idle stop control is executed when the gradient of the road surface is small.
Since this prevents the idle stop control from being executed when the road surface has a large upward gradient in the traveling direction, it is possible to suppress a backward movement of the vehicle at startup and execute the idle stop control to improve fuel economy when the gradient of the road surface is small.